1. Technical Field
The present invention relates to a light-emitting element, a light-emitting device, a display device, and an electronic apparatus.
2. Related Art
An organic electroluminescence element (so-called organic EL element) is a light-emitting element having a structure in which a luminescent organic layer of at least one layer is interposed between an anode and a cathode. In such a light-emitting element, when an electric field is applied between a cathode and an anode, electrons are injected into a light-emitting layer from the cathode and holes are injected into the light-emitting layer from the anode, and thus the electrons and holes are recombined in the light-emitting layer, that is, carriers are recombined to generate excitons. When these excitons return to a ground state, the energy content is emitted as light.
Such an organic El element is thin and lightweight, and is thus proposed to be applied to various illuminations and thin displays.
Here, in illumination applications, since it is necessary to emit white light containing RGB light (three primary colors of light), it is required to use a white light-emitting organic EL element provided with a plurality of different light-emitting layers responsible for emission of each color of RGB in one organic EL element.
Meanwhile, in display applications, in order to obtain an RGB light emission for each pixel, a method of forming organic EL elements having different structures and respectively emitting RGB by patterning is used. However, this method is problematic in that it is difficult to manufacture a fine mask for patterning a light-emitting layer including an organic EL element, and it is difficult to perform a position alignment in the process of manufacturing an organic EL panel. Therefore, it is difficult to realize this method.
For this reason, even in display applications, similar to illumination applications, a white light-emitting organic EL element provided with a plurality of different light-emitting layers responsible for emission of each color of RGB in one organic El element is widely used. In this case, a method of extracting an RGB emission by uniformly forming white light-emitting organic EL elements on a substrate and changing the optical path length around the substrate with respect to each RGB to form different optical resonator structures or to form color filters is used.
As such, when the respective light-emitting layers of RGB are embedded in one organic EL element, it is required to obtain well-balanced light emission from all the RGB light-emitting layers, and it is required to devise a carrier transporting layer made of an emission host material or an intermediate layer provided between light-emitting layers.
For example, JP-A-2005-100921 discloses a light-emitting element for obtaining white light emission, in which a hole transporting layer, a red light-emitting layer, a green light-emitting layer, an intermediate layer, a blue light-emitting layer, and an electron transporting layer are sequentially laminated from an anode side toward a cathode side.
However, in this light-emitting element having such a configuration, the host material used in the blue light-emitting layer has high electron transportability, and thus the recombination regions of electrons and holes are concentrated in the vicinity of the interface between the intermediate layer and the blue light-emitting layer. Therefore, when an electric current is continuously applied, there is a problem in that degradation of the blue light-emitting layer remarkably progresses.